I live in Wisconsin where winters can be quite cold and very humid in the summer.

I have Typar paper over the OSB sheathing. I placed 1" thick foil backed foam board (R=6.5) over the Typar paper. I am currently putting a brick veneer on the whole house. I am tying the brick veneer to the 2x6 wall studs as required and foil taping any seems and holes due to the wall ties. This has taken some time, and is still on going.

I have a couple questions regarding how to properly insulate the interior. With the foil backed insulation on the outside, I don't believe I should put any vapor barrier on the inside regardless of the insulation I will be using on the interior side. Is this correct?

Also, it seems that most people would prefer the closed-cell spray foam insulations if cost wasn't a factor. I understand many of its benefits, but what happens to the moisture that may get between the drywall and the sprayfoam insulation since the wall cavity is most likely not full anyway? Would it not make more sense to use cellulose insulation packed fully into the wall cavity. It seems this would give a more solid feel to the drywall as well as perhaps help with some acoustical properties. Doesn't cellulose also have the ability to absorb some moisture and not lose R-Value?

With the cost of closed cell spray foam being quite high, wouldn't either dense-packed or wet-sprayed cellulose be an equally good alternative?

In a wall assembly, you want to be sure that moisture in the wall can't condense where it can't be controlled. In your cold climate, if your vapor retarder is near the outside of the wall, then you run the risk of condensation where it shouldn't happen. The vapor retarder in a cold climate belongs on the winter warm side of the wall, in your case right behind the wallboard. And you don't want two of them.
What you're doing is incorrect and backwards from the way it should be.

Richard

Is it true that foam sprayed closed cell insulation needs no vapor barrier? If this is the case, what happens to the moisture that gets behind the drywall and in the void area between the drywall and the spray foam insulation? Most closed-cell sprayfoam is cost prohibitive to fill a wall cavity.

I don't understand the reasoning behind what I did as backwards in light of what others say is a benefit about closed cell sprayfoam. You're saying I shouldn't insulate the outside of the house with foil backed rigid insulation? I thought covering the studs from the outside would actually be better than spray-foam, because spray-foam does not give you complete coverage when applied from the inside due to the studs not being covered.

If I were to use spray foam insulation, where does the moisture go then? It has no means of getting out of the wall cavity anyway.

Wouldn't the best thing be to pack the wall cavity full of cellulose and limit any air, which carries moisture, from getting into the wall to begin with?

If you've placed your vapor retarder at the far outside face of the wall, how will moisture escape the wall? It can't. The moisture is generated inside the house in winter, and it tries to migrate from an area of high moisture content to one of lower moisture content. On its way out, if it contacts a cold surface, it will condense on that surface, and run to the bottom of the wall, and collect there, and that starts the cycle of mold and structural decay. It's OK to insulate outside of the studs, but don't place your vapor retarder there. In your wall, the moisture will condense as soon as it hits the backside of the sheathing, and you will have endless and unseen water problems, and more, inside your wall.

There is a temperature gradient in the wall assembly, which you can plot. It can be calculated, but it can also be just estimated. Start with 0 degrees outside the wall, and 70 degrees inside the space, for example. Let's say you lose two degrees in the wallboard. Then maybe 20 degrees in the insulation. That makes it about 48 degrees at the back of the sheathing. If that's below the dewpoint of the air, then that's where the condensation will occur. what happens between that point and the outside doesn't matter any more, you've already got your water where you don't want it. If you stop the vapor directly behind the wallboard, then there is virtually no moisture to condense. That's where your one and only vapor retarder belongs.

This message has been edited. Last edited by: Richard Hetzel,

In winter, moisture is generated inside the house, and tries to migrate from the higher humidity inside to the lower humidity outside. As the moisture migrates through your wall, as soon as it hits a cold surface, it will condense, and the water will run down and collect in the wall, leading to possible mold and structural decay. You don't want that.

In the wall, there is a thermal gradient. Let's assume it's 0 degrees outside, and 70 degrees inside. This gradient can be calculated, but we will just estimate it. Let's say you lose 3 degrees in your wallboard. Then you lose 20 degrees in your insulated space. Now the moisture arrives at the back of the sheathing, where the temperature is 47 degrees. If the temperature of that surface is lower than the dewpoint of the air, condensation occurs right there, and you have water in your wall that cannot escape. Now, if your vapor retarder were located at the back of the wallboard, the moisture goes no further, and because that surface is warm, there is no condensation. THAT is why you want your vapor retarder on the winter warm side of the assembly.

Here is a web site which discusses thermal gradients in walls, for your information:
http://irc.nrc-cnrc.gc.ca/pubs/cbd/cbd036_e.html

Richard

What happens to the moisture between the drywall and the closed-cell sprayed in foam? Where does this moisture go? There is a cavity where moist air can collect, as these are rarely filled solid.

You must think of the moisture in a different light. Its not moisture that enters into the wall at all. Its a vapor. If the temp behind the wall does not go down to the dew point it stays a harmless vapor. Its when the vapor reaches the cooler part of the wall which is the Dew point the water condenses.
Think of it like a glass of ice water. The outside of the glass is where the vapor is. The glass is the vapor barrier. As the ice water cools the glass down what happens. Water that was in a vapor form cools when it touches the glass and it condenses over to water. The same thing happens in the wall. If the vapor goes through the wall and hits the barrier before it cools no moisture condensation happens.
So to answer your question there is no moisture between the drywall and the closed-cell foam.

If you read Richards post this is what he explained to you.

If I use closed-cell foam in the wall-cavity on the inside of the wall and "sandwich" the typar and OSB sheating between the rigid foil faced insulation on the outside, where is the vapor going to go? If it gets through the the claimed vapor barrier of the closed-cell foam, and the OSB and the Typar, wouldn't it just condense on the foil backed insulation, which is seperated from the wall by the typar. The reason foil backed insulation was used to begin with on the outside is that it keeps the wall cavity warmer by covering studs that just spraying closed-cell foam won't, and thus minimizes the amount of condensation that happens "in the wall" to begin with.

What is the purpose of having a vapor barrier for any insulation product behind the drywall if moist air is acceptable in the cavity for closed-cell insulation?

You keep going back to two vapor retarders, which will trap water between them, and that is not what you want. There should be only one, and its proper location in your climate is on the winter warm side of the wall assembly, no matter what the assembly is.

Did you plot the thermal gradient for the wall you want to build? Do you realize that moisture can condense on any surface, even insulation, if the surface temperature is below the dewpoint of the air?

So now how do you want to build your wall? With the spray foam insulation you didn't want to use before? OK, let's estimate the thermal gradient. Start at 70 inside, and work toward 0 degrees outside. The temperature on the back face of the wallboard is 67. Then you have, what? Three and a half inches of air space? The temperature might drop 30 degrees in that space, maybe even more. That makes the temperature no higher than 37 on the inside face of the spray foam. BINGO, you have condensation, and water in your wall, and it's there because there was no vapor retarder where it belongs, and that is directly behind the wallboard.

Throw any combination of ingredients at me, and the same answer will always occur, unless the vapor retarder is where it belongs, and there is only one vapor retarder. We don't just make up these fundamentals. I've explained why they are true, twice now,and given you a link to a paper on the subject. What more do you want from me?

your correct about the foil backed insulation covering the studs. It does bridge the thermal gaps that the studs create. However the foam board is not to have the foil face. Although your theory of sealing both sides of the walls sounds good. In reality it will not work. Any air that is trapped between the two vapor barriers that you created Foam on inside, and foil face on outside will condense and cause issues. The wall requires itself to breath. The typar is not a vapor barrier its a moisture barrier. It allows for breathing of water vapor to travel through should it become trapped in the wall. The foil does not allow this.
Foil faced insulation such as the board stuff your using is NOT to be used as outside wall covering. Its used for basements, on interior walls to cover studs to prevent thermal loss.
Leaks always happen in walls. With the best intentions of the contractor/homeowner. If any water gets behind the outside foil faced wall that is constructed (and it will) you will end up with real long term fungal and rot issues. As well as a thermal loss because of the water that leaked into or condensed within the wall.

Trust us and the many engineers that are much smarter then Richard and I on this.

Do not use the foil faced insulation on the outside wall. Use EPM board that is not faced with any foil product.

Be sure that you have enough between the stud foam insulation area so the wall temps do not go down low enough to allow the surface to go to dew point otherwise you will end up with condensation on the inside part of the wall.

Can't they be just a little smarter??

And just in case you need just one more opinion from someone who has the exact same climate as you.

Do not use foil faced insulation on the cold side of the wall. Double vapor barriers are a serious no-no in our climate. Unfortunately we have one of the worst climates in the country for building in. Wintertime sees vapor from your house trying to get outside and summertime sees it trying to get inside.

And to answer your question about what happens to vapor between the sheetrock and closed cell foam - vapor can move freely through sheetrock most of the time (some people do silly things like put a vapor retardant paint on their sheetrock). So assuming you don't have a condensation point on the surface of your closed cell foam, then the vapor will freely move in and out of your wall cavity. As mentioned before vapor is harmless unless it condenses. Keep in from condensing and you won't have any issues.

Think your asking a lot!

Just thought I'd pass this link along from the Building Science website. Curious as to what others think about this. Notice where they suggest the vapor barrier should be.

http://www.buildingscience.com/documents/insights/bsi-001-the-perfect-wall

Hello again.

Here is a better link from Building Science to explain vapor barriers in more detail. It seems to contradict many beliefs about what we have been told regarding vapor barriers. It even talks about how to properly insulate in Mineapolis, which is north of where I live in Wisconsin, and Chicago.

According to this link from Building Science, I can, and is preferred, use a class I vapor barrier on the exterior - 1" thick foil backed rigid foam behind the brick-veneer to stop thermal bridging, and a class III vapor retarder on the interior with cellulose or open cell in the cavity. This will prevent the condensation on the interior wall cavity from the outside during the SUMMER due to the lack of a class I vapor barrier behind the drywall, and will minimize the temperature reaching dewpoint on the inside of the OSB sheathing in the wall during the WINTER, thus eliminating much of the concern related to condensation within the wall. It will also allow the wall to dry to the inside. It uses the WUFI package to calculate the results.

I'm still curious as to what others have to say about this, as it contradicts what many have been told about vapor barriers/retarders in cold climates. Am I reading this wrong? Is Building Science an unreliable source?

http://www.buildingscience.com/documents/digests/bsd-10...permeance/main_topic

Can't get the link to open.

The link worked fine for me. It is a very detailed outline of vapor barriers, in concept and in practice. I tried to upload the pics to photobucket, but my Linux/Firefox doesn't seem to like it.

The confusion is coming in, I think from some of the pictures. The author agrees that dual vapor barriers are bad, and says so, but a lot of the illustrations are designed to show any outside moisture entering the wall being allowed to ventilate inside the structure, not exactly what you want in a residence in most locations.

I like the wealth of information provided but would recommend a close and careful reading of the text over skimming it and choosing the illustration that most closely fits your application.

Figures 9 and 10 about halfway down the page seem to fit this discussion the best, showing vapor permeable wrap outside, with a vapor barrier on the inside wall.

I don't see where you are getting your quote from. Anything I read there says that the foam should be rigid extruded, not foil faced. And even then it supposed to be thick enough so that the dew point doesn't fall behind the foam.

Figure 8 best shows what I am doing. It says to use a class I vapor retarder on the exterior.

To see the recommendations, scroll down to Sidebar 2.

"Foil-faced isocyanurate 1/2 thick (R 3.5) installed over a 2x4 frame wall meets requirement #9 in Chicago. Therefore, a kraft-faced batt (Class II vapor retarder) is required on the interior of this assembly.

Foil-faced isocyanurate 1 inch thick (R 6) installed over a 2x6 frame wall (R 19) meets requirement #9 in Minneapolis. Therefore, a kraft-faced batt (Class II vapor retarder) is required on the interior of this assembly.

In Chicago where plywood or OSB exterior sheathing is used, an unfaced fiberglass batt can be installed within the wall cavity and gypsum board painted with latex paint (Class III vapor retarder) is required on the interior of this assembly. If this assembly is moved to Minneapolis, a Class II vapor retarder is required on the interior (a kraft paper faced fiberglass batt)."

I can't seem to find what "requirement #9" is. In any case, there is no way that 1" of foam is going to move the dew point away from that surface, not in Minnesota. What happens to all the vapor condensing on the backside of that foil? Something seems to be missing because they can't seriously believe that kraft facing is going to keep out the vapor in the wall assembly during the winter. That is next to impossible, and I speak from experience.